1. Field of the Invention
The present invention relates generally to satellite communications systems, and more specifically to a cross polarization compensation technique.
2. Description of the Related Art
Orthogonal circular or linear polarization is employed for transmission on satellite links to increase communication capacity. For reception, half and quarter wavelength polarizers are used for separating the received signal into respective polarization components. The discrimination between the polarization components, known as cross polarization discrimination (XPD), is an important consideration as a measure of isolation. However, the XPD value degrades if the satellite links are affected by rainfalls. Cross polarization compensation is required to maintain desired isolation using a mono-polarized downlink beacon, as discussed in "Operational Measurements of a 4/6-GHz Adaptive Polarization Compensation Network Employing Up/Down-Link Correlation Algorithms", R. R. Persinger, et al, IEE Second International Conference on Antenna and Propagation, April 1981, IEE Conference Publication. According to this prior art, the beacon signal is used as error vectors (E.sub.x, E.sub.y) to control the downlink half and quarter wavelength polarizers so that the error vectors of the beacon are reduced to zero. On the other hand, both of the received downlink communication and beacon signals are affected by a combined effect of satellite-induced depolarization (or offset) and medium-induced depolarization. Since the mono-polarized beacon has, under certain conditions, a very low XPD value in comparison with that of the downlink communication signal, controlling the polarizers to reduce the beacon's vectors to zero not only compensates for the medium-induced depolarization, but the satellite-induced depolarization. As a result, the in-phase and orthogonal-phase vectors (E.sub.xd, E.sub.yd) of a cross-polarization of the downlink communication signal with respect to the phase of its copolarization (downlink's main signal which is either clockwise or counterclockwise polarized) are not reduced to zero as shown in FIG. 1 (in which the absolute value of each vector represents the amplitude ratio of the main to orthogonal components of each of the clockwise and counterclockwise polarizations and the angle of each vector indicates the phase difference between such components), while the in-phase and orthogonal-phase vectors (E.sub.x, E.sub.y) of a cross-polarization of the beacon signal with respect to the phase of its copolarization (the beacon's main signal) reduces to zero. Because of the correlation between uplink and downlink signals, the in-phase and orthogonal-phase vectors (E.sub.xu, E.sub.yu) of a cross-polarization of the uplink communication signal with respect to the phase of its copolarization are not reduced to zero as indicated in FIG. 1.